1. Field of the Invention
This invention relates to a method of manufacturing surface relief pattern which are of specified and variable cross-sectional and surface geometry, in solid materials, and more particularly to methods of etching the surface of the solid through the use of selective exposure of a resist coating and subsequent bombardment of the resist and solid substrate with reactive ions. Such variable surface geometry is of particular use in the creation of diffraction type gratings, and particularly in the formation of blazed diffraction systems.
2. Description of the Prior Art
In recent years, there has been a substantial increase in the development of optical systems using optical elements having three-dimensional surface relief patterns. Because of relatively new interest in diffraction gratings with non-uniform geometries, efforts have been made to fabricate this type of grating, but with only limited success.
Fabrication has principally been based on photolithographic techniques using a standard photoresist material. Typically, the photoresist has been used as a recording medium for light which defines a light intensity pattern in the photoresist. A commonly used method is to expose the photoresist material to holographically interfering light, with subsequent development of the photoresist coating. A second method commonly used is to fabricate a so-called "binary" mask, which is a mask which is in some locations absolutely opaque to radiation, and in other locations, absolutely transmissive. This pattern is contact printed on the photoresist layer or holographically exposed on the photoresist layer. The photoresist material is then developed chemically, leaving a surface relief pattern in the photoresist which is directly related to the light intensity pattern allowed by the binary mask. Once the photoresist has been exposed and developed, it may be used as an integral part of the completed transmission medium, or can be used as a mask to transfer the pattern to the substrate using methods such as wet etching, ion milling or reactive ion etching.
As shown in FIG. 1, a binary mask contains a pattern of alternating areas, absolutely non-transmissive of radiation and absolutely transmissive of radiation. For many applications, a surface of variable cross section is preferable. In those cases, however, a high degree of variation in the transmission density of the mask is desired to transfer a similar surface relief pattern of continuously variable geometry to the substrate.
Referring now to FIG. 2, a sample or theoretical surface cross section is disclosed. In this example, it is useful to consider the percentage of light transmission in terms of visible light, to better understand the desired result. For example, at point A on the surface of a variable transmission density mask, the mask would be translucent, transmitting perhaps 80% of the available visible light. At point B, by way of further example, the mask would be absolutely opaque and non-transmissive of any visible light wavelengths. Points C and D are provided as samples of points between a pair of points disposed along the variable portion of a linear gradient in the sample mask. Point C, for example, shows a lesser degree, of slope than point D, indicating that the light transmissivity to visible radiation of the mask at point D is changing more rapidly in relation to the surface area of the mask than at point C.
None of the current methods of manufacturing of blazed surfaces or continuously variable surface relief patterns are adequate to produce such a continuously variable surface relief pattern in the substrate material, although such patterns have been successfully transferred to a photoresist coating ("blazed" is a term typically applied to diffraction gratings). In a blazed surface, the grooves in a diffraction grating are of a controlled shape, which give the diffraction grating unique reflection and refraction properties, typically, reflection of large percentages of incoming light into a particular order for a given wavelength. Because the typical photoresist thickness seldom exceeds two microns, there is a corresponding limitation in the depth of the relief pattern which can be created in the photoresist itself. While greater thicknesses of photoresist are attainable using newer techniques, as the thickness of the photoresist increases, likewise, difficulties in obtaining linearity in the desired surface pattern increase.
The use of holography light for exposure requires at least two coherent beams of light which are interfering. The interference pattern is dependent on a complex interference geometry and the resulting intensity of the interfering light pattern. Because the interference pattern is not fully controllable, it is impossible to form generalized surface relief patterns in the photoresist, without a source of coherent light. Moreover, because the pattern of interfering light is depth dependent, it is not possible to fully control the depth of the obtained surface relief pattern.
Referring now to FIG. 3, a previously known and tried method of solving the problem is presented. In an effort to approximate a blazed surface, binary masks have been used in succession, to progressively etch everincreasing areas of the substrate, through multiple applications and, developments of successive photoresist layers. This requires the fabrication of a multiple series of binary masks, and this requires numerous masking, exposing and fabrication steps. These steps consume substantial time and materials, and increase the possibility of error in the final product. The method generally requires the exposure of the surface with a first mask which contains a masking pattern whose features are defined by the width dimension W. Exposure of the substrate is made utilizing this mask, for a measured period of time. After which the photoresist is developed, the substrate etched, and the remaining unwanted resist removed without affecting the substrate. A second mask with a masking pattern width x is then prepared, to overlay precisely along the, previously exposed, substrate in relation to the first pattern in the substrate and the above steps are repeated. Successive masks of width y and z are likewise applied, the photoresist exposed, resulting in step like overall exposure of, the substrate and a subsequent step like pattern in the surface of the substrate. Of course, it can be readily seen that the sloped surface so created along points A, B, C and D on such substrate are merely an approximation of a smooth slope, with various undesirable transmission properties as well as the limitations outlined above.
Attempts to use variable optical density masks for exposing the photoresist have been attempted, on a limited basis. The attempts have been directed to storing optical images directly in the photoresist and then using the photoresist itself as the relief phase storage medium. However, the procedures to transfer an optical pattern, with its linear characteristics, to the underlying solid material substrate have not been attempted. Because of the inherent non-linear absorption characteristics of photoresist, it has been impossible to obtain precise surface relief pattern depths of more than one micron. Variable optical density masks are used to reproduce an image only, not a desired substrate pattern, and hence, the developing and exposing techniques have been unable to compensate for the inherent non-linear characteristics of the photoresist, particularly at large photoresist thicknesses.
Consequently, a need exists for the development of a simple, reliable method of manufacturing surface relief patterns of variable three-dimensional geometry in a solid material which will result in the ability to obtain a wider range of depths of relief in the surface, allow generalized surface relief patterns to be formed, while avoiding the errors which are inherent in the multiple binary masking techniques, all at reasonable cost and with predictable accuracy.